// Copyright 2022 jeff.li. and/or its affiliates.
/*
 * Acknowledgement: This file originates from CPython.
 * https://github.com/python/cpython/blob/3.8/Python/pystrtod.c
 *
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
/* -*- Mode: C; c-file-style: "python" -*- */

#include <locale.h>
#include <math.h>
#include <mutex>

#include <tbir/runtime/exceptions/exceptions.h>
#include <tbir/runtime/py_commons/dtoa.h>
#include <tbir/runtime/py_commons/pymacro.h>
#include <tbir/runtime/py_commons/pymath.h>
#include <tbir/runtime/py_commons/pyport.h>
#include <tbir/runtime/py_commons/pystrtod.h>
#include <tbir/runtime/uchar_util.h>
#include <tbir/runtime/unfixed_buffer.h>

namespace tbir::runtime::py_builtins {

    /* Case-insensitive string match used for nan and inf detection; t should be
       lower-case.  Returns 1 for a successful match, 0 otherwise. */

    static int case_insensitive_match(const char *s, const char *t) {
        while (*t && UCHAR_TOLOWER(*s) == *t) {
            s++;
            t++;
        }
        return *t ? 0 : 1;
    }

/* _Py_parse_inf_or_nan: Attempt to parse a string of the form "nan", "inf" or
   "infinity", with an optional leading sign of "+" or "-".  On success,
   return the NaN or Infinity as a double and set *endptr to point just beyond
   the successfully parsed portion of the string.  On failure, return -1.0 and
   set *endptr to point to the start of the string. */

#ifndef PY_NO_SHORT_FLOAT_REPR

    double _Py_parse_inf_or_nan(const char *p, char **endptr) {
        double retval;
        const char *s;
        int negate = 0;

        s = p;
        if (*s == '-') {
            negate = 1;
            s++;
        } else if (*s == '+') {
            s++;
        }
        if (case_insensitive_match(s, "inf")) {
            s += 3;
            if (case_insensitive_match(s, "inity"))
                s += 5;
            retval = _Py_dg_infinity(negate);
        } else if (case_insensitive_match(s, "nan")) {
            s += 3;
            retval = _Py_dg_stdnan(negate);
        } else {
            s = p;
            retval = -1.0;
        }
        *endptr = (char *) s;
        return retval;
    }

#else

    double _Py_parse_inf_or_nan(const char* p, char** endptr) {
      double retval;
      const char* s;
      int negate = 0;

      s = p;
      if (*s == '-') {
        negate = 1;
        s++;
      } else if (*s == '+') {
        s++;
      }
      if (case_insensitive_match(s, "inf")) {
        s += 3;
        if (case_insensitive_match(s, "inity"))
          s += 5;
        retval = negate ? -HUGE_VAL : HUGE_VAL;
      }
#ifdef Py_NAN
      else if (case_insensitive_match(s, "nan")) {
        s += 3;
        retval = negate ? -Py_NAN : Py_NAN;
      }
#endif
      else {
        s = p;
        retval = -1.0;
      }
      *endptr = (char*)s;
      return retval;
    }

#endif

/**
 * _PyOS_ascii_strtod:
 * @nptr:    the string to convert to a numeric value.
 * @endptr:  if non-%NULL, it returns the character after
 *           the last character used in the conversion.
 *
 * Converts a string to a #gdouble value.
 * This function behaves like the standard strtod() function
 * does in the C locale. It does this without actually
 * changing the current locale, since that would not be
 * thread-safe.
 *
 * This function is typically used when reading configuration
 * files or other non-user input that should be locale independent.
 * To handle input from the user you should normally use the
 * locale-sensitive system strtod() function.
 *
 * If the correct value would cause overflow, plus or minus %HUGE_VAL
 * is returned (according to the sign of the value), and %ERANGE is
 * stored in %errno. If the correct value would cause underflow,
 * zero is returned and %ERANGE is stored in %errno.
 * If memory allocation fails, %ENOMEM is stored in %errno.
 *
 * This function resets %errno before calling strtod() so that
 * you can reliably detect overflow and underflow.
 *
 * Return value: the #gdouble value.
 **/

#ifndef PY_NO_SHORT_FLOAT_REPR

    static double _PyOS_ascii_strtod(const char *nptr, char **endptr) {
        double result;
        _Py_SET_53BIT_PRECISION_HEADER;

        assert(nptr != NULL);
        /* Set errno to zero, so that we can distinguish zero results
           and underflows */
        errno = 0;

        _Py_SET_53BIT_PRECISION_START;
        result = _Py_dg_strtod(nptr, endptr);
        _Py_SET_53BIT_PRECISION_END;

        if (*endptr == nptr)
            /* string might represent an inf or nan */
            result = _Py_parse_inf_or_nan(nptr, endptr);

        return result;
    }

#else

    /*
       Use system strtod;  since strtod is locale aware, we may
       have to first fix the decimal separator.

       Note that unlike _Py_dg_strtod, the system strtod may not always give
       correctly rounded results.
    */

    static double _PyOS_ascii_strtod(const char* nptr, char** endptr) {
      char* fail_pos;
      double val;
      struct lconv* locale_data;
      const char* decimal_point;
      size_t decimal_point_len;
      const char *p, *decimal_point_pos;
      const char* end = NULL; /* Silence gcc */
      const char* digits_pos = NULL;
      int negate = 0;

      assert(nptr != NULL);

      fail_pos = NULL;

      locale_data = localeconv();
      decimal_point = locale_data->decimal_point;
      decimal_point_len = strlen(decimal_point);

      assert(decimal_point_len != 0);

      decimal_point_pos = NULL;

      /* Parse infinities and nans */
      val = _Py_parse_inf_or_nan(nptr, endptr);
      if (*endptr != nptr)
        return val;

      /* Set errno to zero, so that we can distinguish zero results
         and underflows */
      errno = 0;

      /* We process the optional sign manually, then pass the remainder to
         the system strtod.  This ensures that the result of an underflow
         has the correct sign. (bug #1725)  */
      p = nptr;
      /* Process leading sign, if present */
      if (*p == '-') {
        negate = 1;
        p++;
      } else if (*p == '+') {
        p++;
      }

      /* Some platform strtods accept hex floats; Python shouldn't (at the
         moment), so we check explicitly for strings starting with '0x'. */
      if (*p == '0' && (*(p + 1) == 'x' || *(p + 1) == 'X'))
        goto invalid_string;

      /* Check that what's left begins with a digit or decimal point */
      if (!UCHAR_ISDIGIT(*p) && *p != '.')
        goto invalid_string;

      digits_pos = p;
      if (decimal_point[0] != '.' || decimal_point[1] != 0) {
        /* Look for a '.' in the input; if present, it'll need to be
           swapped for the current locale's decimal point before we
           call strtod.  On the other hand, if we find the current
           locale's decimal point then the input is invalid. */
        while (UCHAR_ISDIGIT(*p))
          p++;

        if (*p == '.') {
          decimal_point_pos = p++;

          /* locate end of number */
          while (UCHAR_ISDIGIT(*p))
            p++;

          if (*p == 'e' || *p == 'E')
            p++;
          if (*p == '+' || *p == '-')
            p++;
          while (UCHAR_ISDIGIT(*p))
            p++;
          end = p;
        } else if (strncmp(p, decimal_point, decimal_point_len) == 0)
          /* Python bug #1417699 */
          goto invalid_string;
        /* For the other cases, we need not convert the decimal
           point */
      }

      if (decimal_point_pos) {
        char *copy, *c;
        /* Create a copy of the input, with the '.' converted to the
           locale-specific decimal point */
        copy = (char*)malloc(end - digits_pos + 1 + decimal_point_len);
        if (copy == NULL) {
          *endptr = (char*)nptr;
          errno = ENOMEM;
          return val;
        }

        c = copy;
        memcpy(c, digits_pos, decimal_point_pos - digits_pos);
        c += decimal_point_pos - digits_pos;
        memcpy(c, decimal_point, decimal_point_len);
        c += decimal_point_len;
        memcpy(c, decimal_point_pos + 1, end - (decimal_point_pos + 1));
        c += end - (decimal_point_pos + 1);
        *c = 0;

        val = strtod(copy, &fail_pos);

        if (fail_pos) {
          if (fail_pos > decimal_point_pos)
            fail_pos = (char*)digits_pos + (fail_pos - copy) - (decimal_point_len - 1);
          else
            fail_pos = (char*)digits_pos + (fail_pos - copy);
        }

        free(copy);

      } else {
        val = strtod(digits_pos, &fail_pos);
      }

      if (fail_pos == digits_pos)
        goto invalid_string;

      if (negate && fail_pos != nptr)
        val = -val;
      *endptr = fail_pos;

      return val;

    invalid_string:
      *endptr = (char*)nptr;
      errno = EINVAL;
      return -1.0;
    }

#endif

    /* PyOS_string_to_double converts a null-terminated byte string s (interpreted
       as a string of ASCII characters) to a float.  The string should not have
       leading or trailing whitespace.  The conversion is independent of the
       current locale.

       If endptr is NULL, try to convert the whole string.  Raise ValueError and
       return -1.0 if the string is not a valid representation of a floating-point
       number.

       If endptr is non-NULL, try to convert as much of the string as possible.
       If no initial segment of the string is the valid representation of a
       floating-point number then *endptr is set to point to the beginning of the
       string, -1.0 is returned and again ValueError is raised.

       On overflow (e.g., when trying to convert '1e500' on an IEEE 754 machine),
       if overflow_exception is NULL then +-Py_HUGE_VAL is returned, and no Python
       exception is raised.  Otherwise, overflow_exception should point to
       a Python exception, this exception will be raised, -1.0 will be returned,
       and *endptr will point just past the end of the converted value.

       If any other failure occurs (for example lack of memory), -1.0 is returned
       and the appropriate Python exception will have been set.
    */

    double PyOS_string_to_double(const char *s, char **endptr) {
        double x, result = -1.0;
        char *fail_pos;

        errno = 0;
        x = _PyOS_ascii_strtod(s, &fail_pos);

        if (errno == ENOMEM) {
            THROW_PY_MemoryError("could not convert string to float: ", s);
            fail_pos = (char *) s;
        } else if (!endptr && (fail_pos == s || *fail_pos != '\0')) {
            THROW_PY_ValueError("could not convert string to float: ", s);
        } else if (fail_pos == s) {
            THROW_PY_ValueError("could not convert string to float: ", s);
        } else if (errno == ERANGE && fabs(x) >= 1.0) {
            THROW_PY_OverflowError("value too large to convert to float: ", s);
        } else {
            result = x;
        }

        if (endptr != NULL) {
            *endptr = fail_pos;
        }

        return result;
    }

#ifdef PY_NO_SHORT_FLOAT_REPR

    /* Given a string that may have a decimal point in the current
       locale, change it back to a dot.  Since the string cannot get
       longer, no need for a maximum buffer size parameter. */
    static inline void change_decimal_from_locale_to_dot(char* buffer) {
      struct lconv* locale_data = localeconv();
      const char* decimal_point = locale_data->decimal_point;

      if (decimal_point[0] != '.' || decimal_point[1] != 0) {
        size_t decimal_point_len = strlen(decimal_point);

        if (*buffer == '+' || *buffer == '-')
          buffer++;
        while (UCHAR_ISDIGIT(*buffer))
          buffer++;
        if (strncmp(buffer, decimal_point, decimal_point_len) == 0) {
          *buffer = '.';
          buffer++;
          if (decimal_point_len > 1) {
            /* buffer needs to get smaller */
            size_t rest_len = strlen(buffer + (decimal_point_len - 1));
            memmove(buffer, buffer + (decimal_point_len - 1), rest_len);
            buffer[rest_len] = 0;
          }
        }
      }
    }

    /* From the C99 standard, section 7.19.6:
    The exponent always contains at least two digits, and only as many more digits
    as necessary to represent the exponent.
    */
#define MIN_EXPONENT_DIGITS 2

    /* Ensure that any exponent, if present, is at least MIN_EXPONENT_DIGITS
       in length. */
    static inline void ensure_minimum_exponent_length(char* buffer, size_t buf_size) {
      char* p = strpbrk(buffer, "eE");
      if (p && (*(p + 1) == '-' || *(p + 1) == '+')) {
        char* start = p + 2;
        int exponent_digit_cnt = 0;
        int leading_zero_cnt = 0;
        int in_leading_zeros = 1;
        int significant_digit_cnt;

        /* Skip over the exponent and the sign. */
        p += 2;

        /* Find the end of the exponent, keeping track of leading
           zeros. */
        while (*p && UCHAR_ISDIGIT(*p)) {
          if (in_leading_zeros && *p == '0')
            ++leading_zero_cnt;
          if (*p != '0')
            in_leading_zeros = 0;
          ++p;
          ++exponent_digit_cnt;
        }

        significant_digit_cnt = exponent_digit_cnt - leading_zero_cnt;
        if (exponent_digit_cnt == MIN_EXPONENT_DIGITS) {
          /* If there are 2 exactly digits, we're done,
             regardless of what they contain */
        } else if (exponent_digit_cnt > MIN_EXPONENT_DIGITS) {
          int extra_zeros_cnt;

          /* There are more than 2 digits in the exponent.  See
             if we can delete some of the leading zeros */
          if (significant_digit_cnt < MIN_EXPONENT_DIGITS)
            significant_digit_cnt = MIN_EXPONENT_DIGITS;
          extra_zeros_cnt = exponent_digit_cnt - significant_digit_cnt;

          /* Delete extra_zeros_cnt worth of characters from the
             front of the exponent */
          assert(extra_zeros_cnt >= 0);

          /* Add one to significant_digit_cnt to copy the
             trailing 0 byte, thus setting the length */
          memmove(start, start + extra_zeros_cnt, significant_digit_cnt + 1);
        } else {
          /* If there are fewer than 2 digits, add zeros
             until there are 2, if there's enough room */
          int zeros = MIN_EXPONENT_DIGITS - exponent_digit_cnt;
          if (start + zeros + exponent_digit_cnt + 1 < buffer + buf_size) {
            memmove(start + zeros, start, exponent_digit_cnt + 1);
            memset(start, '0', zeros);
          }
        }
      }
    }

    /* Remove trailing zeros after the decimal point from a numeric string; also
       remove the decimal point if all digits following it are zero.  The numeric
       string must end in '\0', and should not have any leading or trailing
       whitespace.  Assumes that the decimal point is '.'. */
    static inline void remove_trailing_zeros(char* buffer) {
      char *old_fraction_end, *new_fraction_end, *end, *p;

      p = buffer;
      if (*p == '-' || *p == '+')
        /* Skip leading sign, if present */
        ++p;
      while (UCHAR_ISDIGIT(*p))
        ++p;

      /* if there's no decimal point there's nothing to do */
      if (*p++ != '.')
        return;

      /* scan any digits after the point */
      while (UCHAR_ISDIGIT(*p))
        ++p;
      old_fraction_end = p;

      /* scan up to ending '\0' */
      while (*p != '\0')
        p++;
      /* +1 to make sure that we move the null byte as well */
      end = p + 1;

      /* scan back from fraction_end, looking for removable zeros */
      p = old_fraction_end;
      while (*(p - 1) == '0')
        --p;
      /* and remove point if we've got that far */
      if (*(p - 1) == '.')
        --p;
      new_fraction_end = p;

      memmove(new_fraction_end, old_fraction_end, end - old_fraction_end);
    }

    /* Ensure that buffer has a decimal point in it.  The decimal point will not
       be in the current locale, it will always be '.'. Don't add a decimal point
       if an exponent is present.  Also, convert to exponential notation where
       adding a '.0' would produce too many significant digits (see issue 5864).

       Returns a pointer to the fixed buffer, or NULL on failure.
    */
    static inline char* ensure_decimal_point(char* buffer, size_t buf_size, int precision) {
      int digit_count, insert_count = 0, convert_to_exp = 0;
      const char* chars_to_insert;
      char* digits_start;

      /* search for the first non-digit character */
      char* p = buffer;
      if (*p == '-' || *p == '+')
        /* Skip leading sign, if present.  I think this could only
           ever be '-', but it can't hurt to check for both. */
        ++p;
      digits_start = p;
      while (*p && UCHAR_ISDIGIT(*p))
        ++p;
      digit_count = Py_SAFE_DOWNCAST(p - digits_start, intptr_t, int);

      if (*p == '.') {
        if (UCHAR_ISDIGIT(*(p + 1))) {
          /* Nothing to do, we already have a decimal
             point and a digit after it */
        } else {
          /* We have a decimal point, but no following
             digit.  Insert a zero after the decimal. */
          /* can't ever get here via PyOS_double_to_string */
          assert(precision == -1);
          ++p;
          chars_to_insert = "0";
          insert_count = 1;
        }
      } else if (!(*p == 'e' || *p == 'E')) {
        /* Don't add ".0" if we have an exponent. */
        if (digit_count == precision) {
          /* issue 5864: don't add a trailing .0 in the case
             where the '%g'-formatted result already has as many
             significant digits as were requested.  Switch to
             exponential notation instead. */
          convert_to_exp = 1;
          /* no exponent, no point, and we shouldn't land here
             for infs and nans, so we must be at the end of the
             string. */
          assert(*p == '\0');
        } else {
          assert(precision == -1 || digit_count < precision);
          chars_to_insert = ".0";
          insert_count = 2;
        }
      }
      if (insert_count) {
        size_t buf_len = strlen(buffer);
        if (buf_len + insert_count + 1 >= buf_size) {
          /* If there is not enough room in the buffer
             for the additional text, just skip it.  It's
             not worth generating an error over. */
        } else {
          memmove(p + insert_count, p, buffer + strlen(buffer) - p + 1);
          memcpy(p, chars_to_insert, insert_count);
        }
      }
      if (convert_to_exp) {
        int written;
        size_t buf_avail;
        p = digits_start;
        /* insert decimal point */
        assert(digit_count >= 1);
        memmove(p + 2, p + 1, digit_count); /* safe, but overwrites nul */
        p[1] = '.';
        p += digit_count + 1;
        assert(p <= buf_size + buffer);
        buf_avail = buf_size + buffer - p;
        if (buf_avail == 0)
          return NULL;
        /* Add exponent.  It's okay to use lower case 'e': we only
           arrive here as a result of using the empty format code or
           repr/str builtins and those never want an upper case 'E' */
        written = snprintf(p, buf_avail, "e%+.02d", digit_count - 1);
        if (!(0 <= written && written < Py_SAFE_DOWNCAST(buf_avail, size_t, int)))
          /* output truncated, or something else bad happened */
          return NULL;
        remove_trailing_zeros(buffer);
      }
      return buffer;
    }

    /* see FORMATBUFLEN in unicodeobject.c */
#define FLOAT_FORMATBUFLEN 120

    /**
     * _PyOS_ascii_formatd:
     * @buffer: A buffer to place the resulting string in
     * @buf_size: The length of the buffer.
     * @format: The printf()-style format to use for the
     *          code to use for converting.
     * @d: The #gdouble to convert
     * @precision: The precision to use when formatting.
     *
     * Converts a #gdouble to a string, using the '.' as
     * decimal point. To format the number you pass in
     * a printf()-style format string. Allowed conversion
     * specifiers are 'e', 'E', 'f', 'F', 'g', 'G', and 'Z'.
     *
     * 'Z' is the same as 'g', except it always has a decimal and
     *     at least one digit after the decimal.
     *
     * Return value: The pointer to the buffer with the converted string.
     * On failure returns NULL but does not set any Python exception.
     **/
    static char* _PyOS_ascii_formatd(
        char* buffer, size_t buf_size, const char* format, double d, int precision) {
      char format_char;
      size_t format_len = strlen(format);

      /* Issue 2264: code 'Z' requires copying the format.  'Z' is 'g', but
         also with at least one character past the decimal. */
      char tmp_format[FLOAT_FORMATBUFLEN];

      /* The last character in the format string must be the format char */
      format_char = format[format_len - 1];

      if (format[0] != '%')
        return NULL;

      /* I'm not sure why this test is here.  It's ensuring that the format
         string after the first character doesn't have a single quote, a
         lowercase l, or a percent. This is the reverse of the commented-out
         test about 10 lines ago. */
      if (strpbrk(format + 1, "'l%"))
        return NULL;

      /* Also curious about this function is that it accepts format strings
         like "%xg", which are invalid for floats.  In general, the
         interface to this function is not very good, but changing it is
         difficult because it's a public API. */

      if (!(format_char == 'e' || format_char == 'E' || format_char == 'f' || format_char == 'F' ||
            format_char == 'g' || format_char == 'G' || format_char == 'Z'))
        return NULL;

      /* Map 'Z' format_char to 'g', by copying the format string and
         replacing the final char with a 'g' */
      if (format_char == 'Z') {
        if (format_len + 1 >= sizeof(tmp_format)) {
          /* The format won't fit in our copy.  Error out.  In
             practice, this will never happen and will be
             detected by returning NULL */
          return NULL;
        }
        strcpy(tmp_format, format);
        tmp_format[format_len - 1] = 'g';
        format = tmp_format;
      }

      /* Have PyOS_snprintf do the hard work */
      snprintf(buffer, buf_size, format, d);

      /* Do various fixups on the return string */

      /* Get the current locale, and find the decimal point string.
         Convert that string back to a dot. */
      change_decimal_from_locale_to_dot(buffer);

      /* If an exponent exists, ensure that the exponent is at least
         MIN_EXPONENT_DIGITS digits, providing the buffer is large enough
         for the extra zeros.  Also, if there are more than
         MIN_EXPONENT_DIGITS, remove as many zeros as possible until we get
         back to MIN_EXPONENT_DIGITS */
      ensure_minimum_exponent_length(buffer, buf_size);

      /* If format_char is 'Z', make sure we have at least one character
         after the decimal point (and make sure we have a decimal point);
         also switch to exponential notation in some edge cases where the
         extra character would produce more significant digits that we
         really want. */
      if (format_char == 'Z')
        buffer = ensure_decimal_point(buffer, buf_size, precision);

      return buffer;
    }

    /* The fallback code to use if _Py_dg_dtoa is not available. */

    String PyOS_double_to_string(double val, char format_code, int precision, int flags, int* type) {
      char format[32];
      intptr_t bufsize;
      char* buf;
      int t, exp;
      int upper = 0;
      UnfixedBuffer<char, 256> mixed_buf;

      /* Validate format_code, and map upper and lower case */
      switch (format_code) {
        case 'e': /* exponent */
        case 'f': /* fixed */
        case 'g': /* general */
          break;
        case 'E':
          upper = 1;
          format_code = 'e';
          break;
        case 'F':
          upper = 1;
          format_code = 'f';
          break;
        case 'G':
          upper = 1;
          format_code = 'g';
          break;
        case 'r': /* repr format */
          /* Supplied precision is unused, must be 0. */
          if (precision != 0) {
            THROW_PY_SystemError("PyOS_double_to_string: BadInternalCall");
            return String{};
          }
          /* The repr() precision (17 significant decimal digits) is the
             minimal number that is guaranteed to have enough precision
             so that if the number is read back in the exact same binary
             value is recreated.  This is true for IEEE floating point
             by design, and also happens to work for all other modern
             hardware. */
          precision = 17;
          format_code = 'g';
          break;
        default:
          THROW_PY_SystemError("PyOS_double_to_string: BadInternalCall");
          return String{};
      }

      /* Here's a quick-and-dirty calculation to figure out how big a buffer
         we need.  In general, for a finite float we need:

           1 byte for each digit of the decimal significand, and

           1 for a possible sign
           1 for a possible decimal point
           2 for a possible [eE][+-]
           1 for each digit of the exponent;  if we allow 19 digits
             total then we're safe up to exponents of 2**63.
           1 for the trailing nul byte

         This gives a total of 24 + the number of digits in the significand,
         and the number of digits in the significand is:

           for 'g' format: at most precision, except possibly
             when precision == 0, when it's 1.
           for 'e' format: precision+1
           for 'f' format: precision digits after the point, at least 1
             before.  To figure out how many digits appear before the point
             we have to examine the size of the number.  If fabs(val) < 1.0
             then there will be only one digit before the point.  If
             fabs(val) >= 1.0, then there are at most

           1+floor(log10(ceiling(fabs(val))))

             digits before the point (where the 'ceiling' allows for the
             possibility that the rounding rounds the integer part of val
             up).  A safe upper bound for the above quantity is
             1+floor(exp/3), where exp is the unique integer such that 0.5
             <= fabs(val)/2**exp < 1.0.  This exp can be obtained from
             frexp.

         So we allow room for precision+1 digits for all formats, plus an
         extra floor(exp/3) digits for 'f' format.

      */

      if (isnan(val) || isinf(val))
        /* 3 for 'inf'/'nan', 1 for sign, 1 for '\0' */
        bufsize = 5;
      else {
        bufsize = 25 + precision;
        if (format_code == 'f' && fabs(val) >= 1.0) {
          frexp(val, &exp);
          bufsize += exp / 3;
        }
      }

      buf = mixed_buf.Data(bufsize);
      if (buf == NULL) {
        THROW_PY_MemoryError("PyOS_double_to_string: no memory");
        return String{};
      }

      /* Handle nan and inf. */
      if (isnan(val)) {
        strcpy(buf, "nan");
        t = Py_DTST_NAN;
      } else if (isinf(val)) {
        if (copysign(1., val) == 1.)
          strcpy(buf, "inf");
        else
          strcpy(buf, "-inf");
        t = Py_DTST_INFINITE;
      } else {
        t = Py_DTST_FINITE;
        if (flags & Py_DTSF_ADD_DOT_0)
          format_code = 'Z';

        snprintf(format,
                 sizeof(format),
                 "%%%s.%i%c",
                 (flags & Py_DTSF_ALT ? "#" : ""),
                 precision,
                 format_code);
        _PyOS_ascii_formatd(buf, bufsize, format, val, precision);
      }

      /* Add sign when requested.  It's convenient (esp. when formatting
       complex numbers) to include a sign even for inf and nan. */
      if (flags & Py_DTSF_SIGN && buf[0] != '-') {
        size_t len = strlen(buf);
        /* the bufsize calculations above should ensure that we've got
           space to add a sign */
        assert((size_t)bufsize >= len + 2);
        memmove(buf + 1, buf, len + 1);
        buf[0] = '+';
      }
      if (upper) {
        /* Convert to upper case. */
        char* p1;
        for (p1 = buf; *p1; p1++)
          *p1 = UCHAR_TOUPPER(*p1);
      }

      if (type)
        *type = t;
      return String(buf);
    }

#else

/* _Py_dg_dtoa is available. */

/* I'm using a lookup table here so that I don't have to invent a non-locale
   specific way to convert to uppercase */
#define OFS_INF 0
#define OFS_NAN 1
#define OFS_E 2

    /* The lengths of these are known to the code below, so don't change them */
    static const char *const lc_float_strings[] = {
            "inf",
            "nan",
            "e",
    };
    static const char *const uc_float_strings[] = {
            "INF",
            "NAN",
            "E",
    };

    /* Convert a double d to a string, and return a
       string contain the resulting string.

       Arguments:
         d is the double to be converted
         format_code is one of 'e', 'f', 'g', 'r'.  'e', 'f' and 'g'
           correspond to '%e', '%f' and '%g';  'r' corresponds to repr.
         mode is one of '0', '2' or '3', and is completely determined by
           format_code: 'e' and 'g' use mode 2; 'f' mode 3, 'r' mode 0.
         precision is the desired precision
         always_add_sign is nonzero if a '+' sign should be included for positive
           numbers
         add_dot_0_if_integer is nonzero if integers in non-exponential form
           should have ".0" added.  Only applies to format codes 'r' and 'g'.
         use_alt_formatting is nonzero if alternative formatting should be
           used.  Only applies to format codes 'e', 'f' and 'g'.  For code 'g',
           at most one of use_alt_formatting and add_dot_0_if_integer should
           be nonzero.
         type, if non-NULL, will be set to one of these constants to identify
           the type of the 'd' argument:
         Py_DTST_FINITE
         Py_DTST_INFINITE
         Py_DTST_NAN

       Returns a string containing the resulting string,
        or NULL on error. If NULL is returned, the Python error has been set.
     */

    static String format_float_short(double d,
                                     char format_code,
                                     int mode,
                                     int precision,
                                     int always_add_sign,
                                     int add_dot_0_if_integer,
                                     int use_alt_formatting,
                                     const char *const *float_strings,
                                     int *type) {
        char *buf = nullptr;
        char *p = nullptr;
        int64_t bufsize = 0;
        char *digits, *digits_end;
        int decpt_as_int, sign, exp_len, exp = 0, use_exp = 0;
        int64_t decpt, digits_len, vdigits_start, vdigits_end;
        UnfixedBuffer<char, 256> mixed_buf;
        _Py_SET_53BIT_PRECISION_HEADER;

        /* _Py_dg_dtoa returns a digit string (no decimal point or exponent).
           Must be matched by a call to _Py_dg_freedtoa. */
        _Py_SET_53BIT_PRECISION_START;
        digits = _Py_dg_dtoa(d, mode, precision, &decpt_as_int, &sign, &digits_end);
        _Py_SET_53BIT_PRECISION_END;

        decpt = (intptr_t) decpt_as_int;
        if (digits == NULL) {
            /* The only failure mode is no memory. */
            THROW_PY_MemoryError("format_float_short: no memory");
            goto exit;
        }
        assert(digits_end != NULL && digits_end >= digits);
        digits_len = digits_end - digits;

        if (digits_len && !UCHAR_ISDIGIT(digits[0])) {
            /* Infinities and nans here; adapt Gay's output,
               so convert Infinity to inf and NaN to nan, and
               ignore sign of nan. Then return. */

            /* ignore the actual sign of a nan */
            if (digits[0] == 'n' || digits[0] == 'N')
                sign = 0;

            /* We only need 5 bytes to hold the result "+inf\0" . */
            bufsize = 5; /* Used later in an assert. */
            buf = mixed_buf.Data(bufsize);
            if (buf == NULL) {
                THROW_PY_MemoryError("format_float_short: no memory");
                goto exit;
            }
            p = buf;

            if (sign == 1) {
                *p++ = '-';
            } else if (always_add_sign) {
                *p++ = '+';
            }
            if (digits[0] == 'i' || digits[0] == 'I') {
                strncpy(p, float_strings[OFS_INF], 3);
                p += 3;

                if (type)
                    *type = Py_DTST_INFINITE;
            } else if (digits[0] == 'n' || digits[0] == 'N') {
                strncpy(p, float_strings[OFS_NAN], 3);
                p += 3;

                if (type)
                    *type = Py_DTST_NAN;
            } else {
                /* shouldn't get here: Gay's code should always return
                   something starting with a digit, an 'I',  or 'N' */
                Py_UNREACHABLE();
            }
            goto exit;
        }

        /* The result must be finite (not inf or nan). */
        if (type)
            *type = Py_DTST_FINITE;

        /* We got digits back, format them.  We may need to pad 'digits'
           either on the left or right (or both) with extra zeros, so in
           general the resulting string has the form

             [<sign>]<zeros><digits><zeros>[<exponent>]

           where either of the <zeros> pieces could be empty, and there's a
           decimal point that could appear either in <digits> or in the
           leading or trailing <zeros>.

           Imagine an infinite 'virtual' string vdigits, consisting of the
           string 'digits' (starting at index 0) padded on both the left and
           right with infinite strings of zeros.  We want to output a slice

             vdigits[vdigits_start : vdigits_end]

           of this virtual string.  Thus if vdigits_start < 0 then we'll end
           up producing some leading zeros; if vdigits_end > digits_len there
           will be trailing zeros in the output.  The next section of code
           determines whether to use an exponent or not, figures out the
           position 'decpt' of the decimal point, and computes 'vdigits_start'
           and 'vdigits_end'. */
        vdigits_end = digits_len;
        switch (format_code) {
            case 'e':
                use_exp = 1;
                vdigits_end = precision;
                break;
            case 'f':
                vdigits_end = decpt + precision;
                break;
            case 'g':
                if (decpt <= -4 || decpt > (add_dot_0_if_integer ? precision - 1 : precision))
                    use_exp = 1;
                if (use_alt_formatting)
                    vdigits_end = precision;
                break;
            case 'r':
                /* convert to exponential format at 1e16.  We used to convert
                   at 1e17, but that gives odd-looking results for some values
                   when a 16-digit 'shortest' repr is padded with bogus zeros.
                   For example, repr(2e16+8) would give 20000000000000010.0;
                   the true value is 20000000000000008.0. */
                if (decpt <= -4 || decpt > 16)
                    use_exp = 1;
                break;
            default:
                THROW_PY_SystemError("format_float_short: BadInternalCall");
                goto exit;
        }

        /* if using an exponent, reset decimal point position to 1 and adjust
           exponent accordingly.*/
        if (use_exp) {
            exp = (int) decpt - 1;
            decpt = 1;
        }
        /* ensure vdigits_start < decpt <= vdigits_end, or vdigits_start <
           decpt < vdigits_end if add_dot_0_if_integer and no exponent */
        vdigits_start = decpt <= 0 ? decpt - 1 : 0;
        if (!use_exp && add_dot_0_if_integer)
            vdigits_end = vdigits_end > decpt ? vdigits_end : decpt + 1;
        else
            vdigits_end = vdigits_end > decpt ? vdigits_end : decpt;

        /* double check inequalities */
        assert(vdigits_start <= 0 && 0 <= digits_len && digits_len <= vdigits_end);
        /* decimal point should be in (vdigits_start, vdigits_end] */
        assert(vdigits_start < decpt && decpt <= vdigits_end);

        /* Compute an upper bound how much memory we need. This might be a few
           chars too long, but no big deal. */
        bufsize =
                /* sign, decimal point and trailing 0 byte */
                3 +

                /* total digit count (including zero padding on both sides) */
                (vdigits_end - vdigits_start) +

                /* exponent "e+100", max 3 numerical digits */
                (use_exp ? 5 : 0);

        /* Now allocate the memory and initialize p to point to the start of
           it. */
        buf = mixed_buf.Data(bufsize);
        if (buf == NULL) {
            THROW_PY_MemoryError("format_float_short: no memory");
            goto exit;
        }
        p = buf;

        /* Add a negative sign if negative, and a plus sign if non-negative
           and always_add_sign is true. */
        if (sign == 1)
            *p++ = '-';
        else if (always_add_sign)
            *p++ = '+';

        /* note that exactly one of the three 'if' conditions is true,
           so we include exactly one decimal point */
        /* Zero padding on left of digit string */
        if (decpt <= 0) {
            memset(p, '0', decpt - vdigits_start);
            p += decpt - vdigits_start;
            *p++ = '.';
            memset(p, '0', 0 - decpt);
            p += 0 - decpt;
        } else {
            memset(p, '0', 0 - vdigits_start);
            p += 0 - vdigits_start;
        }

        /* Digits, with included decimal point */
        if (0 < decpt && decpt <= digits_len) {
            strncpy(p, digits, decpt - 0);
            p += decpt - 0;
            *p++ = '.';
            strncpy(p, digits + decpt, digits_len - decpt);
            p += digits_len - decpt;
        } else {
            strncpy(p, digits, digits_len);
            p += digits_len;
        }

        /* And zeros on the right */
        if (digits_len < decpt) {
            memset(p, '0', decpt - digits_len);
            p += decpt - digits_len;
            *p++ = '.';
            memset(p, '0', vdigits_end - decpt);
            p += vdigits_end - decpt;
        } else {
            memset(p, '0', vdigits_end - digits_len);
            p += vdigits_end - digits_len;
        }

        /* Delete a trailing decimal pt unless using alternative formatting. */
        if (p[-1] == '.' && !use_alt_formatting)
            p--;

        /* Now that we've done zero padding, add an exponent if needed. */
        if (use_exp) {
            *p++ = float_strings[OFS_E][0];
            exp_len = sprintf(p, "%+.02d", exp);
            p += exp_len;
        }
        exit:
        String r;
        if (buf) {
            *p = '\0';
            r.append(buf);
            /* It's too late if this fails, as we've already stepped on
               memory that isn't ours. But it's an okay debugging test. */
            assert(p - buf < bufsize);
        }
        if (digits)
            _Py_dg_freedtoa(digits);

        return r;
    }

    static std::mutex PyOS_double_to_string_lock;

    String PyOS_double_to_string(double val, char format_code, int precision, int flags, int *type) {
        const char *const *float_strings = lc_float_strings;
        int mode;
        std::lock_guard<std::mutex> lock_guard(PyOS_double_to_string_lock);

        /* Validate format_code, and map upper and lower case. Compute the
           mode and make any adjustments as needed. */
        switch (format_code) {
            /* exponent */
            case 'E':
                float_strings = uc_float_strings;
                format_code = 'e';
                /* Fall through. */
            case 'e':
                mode = 2;
                precision++;
                break;

                /* fixed */
            case 'F':
                float_strings = uc_float_strings;
                format_code = 'f';
                /* Fall through. */
            case 'f':
                mode = 3;
                break;

                /* general */
            case 'G':
                float_strings = uc_float_strings;
                format_code = 'g';
                /* Fall through. */
            case 'g':
                mode = 2;
                /* precision 0 makes no sense for 'g' format; interpret as 1 */
                if (precision == 0)
                    precision = 1;
                break;

                /* repr format */
            case 'r':
                mode = 0;
                /* Supplied precision is unused, must be 0. */
                if (precision != 0) {
                    THROW_PY_SystemError("bad argument to internal function");
                    return "";
                }
                break;

            default:
                THROW_PY_SystemError("bad argument to internal function");
                return "";
        }

        return format_float_short(val,
                                  format_code,
                                  mode,
                                  precision,
                                  flags & Py_DTSF_SIGN,
                                  flags & Py_DTSF_ADD_DOT_0,
                                  flags & Py_DTSF_ALT,
                                  float_strings,
                                  type);
    }

#endif /* ifdef PY_NO_SHORT_FLOAT_REPR */

}  // namespace tbir::runtime::py_builtins
